A current sensing resistor, when serially connected to a load and applied current thereto, results in a voltage drop which may be measured and referred to estimate the current intensity. Since the resistance of a current sensing resistor is generally at a milliohm (mOhm) order, high resistance precision, e.g. with deviation within ±1%, is required compared to a common resistor. Accordingly, proper adjustment is generally performed in the manufacturing process of the current sensing resistor after measuring resistance of the newly produced resistor and calculating deviation of the measured resistance from a preset ideal value. Repetitive measurement and adjustment are performed until the measured resistance is close enough to the preset ideal value.
Conventionally, Kelvin measurement, which is a four-point type of measurement, is adopted to measure resistance of a current sensing resistor. The principle will be described hereinafter.
Please refer to FIG. 1, which schematically illustrates circuitry associated with Kelvin measurement. As shown, two ends of a resistor 15 whose resistance R is to be measured are respectively connected to four points 11, 12, 13 and 14. The points 13 and 14 are further respectively connected to head and tail ends of a constant current source 16 which supplies a constant current intensity I. On the other hand, the points 11 and 12 are coupled to respective probes with high impedance for measuring voltage difference therebetween. Since the input impedance of the probes coupled to the points 11 and 12 is relative high, it is assumed that no current would pass through point 11, resistor 15 and point 12, i.e. i1=0i2=0. Under this circumstance, the constant current source 16, point 14, resistor 15 and point 13 form a circuit loop, and the voltage difference V between the points 11 and 12, where V=V11−V12, can be measured and used for calculating resistance of the resistor 15 based on Ohm's Law, i.e. V=IR.
FIG. 2A illustrates a structure of a conventional current sensing resistor. The current sensing resistor 100 includes a resistor plate 120 and two electrode plates 110 and 130 respectively welded to opposite sides of the resistor plate 120 and having apertures 140 and 150. On the electrode plates, sensing pads 111 and 113 and current pads 112 and 132 are defined as measuring area. When producing the current sensing resistor 100, a constant current I is applied between the current pads 112 and 132, and a voltage difference rendered between the sensing pads 111 and 131 (Vdiff=V111−V131) when the constant current I passes through the current sensing resistor 100 is measured. Accordingly, resistance R1 of the resistor 120 can be calculated as R1=Vdiff/I.
Please refer to FIG. 2B, which illustrates four measurement points defined in a measuring apparatus for measuring resistance of a newly produced resistor. The four measurement points 211, 212, 213 and 214 are arranged on the electrode plate in zones 311, 312, 313 and 314 as a rectangle, as shown in FIG. 2C, wherein the measurement points 213 and 214 are associated with constant current input and the measurement points 211 and 212 are associated with output voltage measurement.
The newly produced resistor is then performed with a barrel plating process to be electroplated with a soldering layer for facilitating the mounting of the resistor onto a printed circuit board (PCB).
Since the soldering layer is applied onto the resistor after the measurement and adjustment of the resistance of the resistor has been accomplished, the resistor product suffers from resistance deviation due to the newly applied soldering layer. Furthermore, the resistors produced in a batch would have diverse resistance. For example, as illustrated in FIG. 3A, the resistance of resistors produced in a batch before the soldering layer is applied is generally distributed within a narrow range, e.g. ±0.8% from the peak value. However, after the soldering layer is applied, the resistance of the resistors becomes diverse as distributed within a wide range, e.g. ±3% from the peak value, as illustrated in FIG. 3B. Thus the requirement on high resistance precision cannot be complied with.